A traditional solid-state detector type to be used for X-ray detection in e.g. imaging and spectroscopy applications is the PIN detector. There the detecting element is a reverse biased PIN diode, one electrode of which is coupled to the gate of a FET (field-effect transistor). X-ray quanta that hit the PIN diode cause a photoelectric effect, creating a number of free electrons and holes in a depleted region formed in the semiconductor material. The bias voltage across the PIN diode causes the mobile charge carriers to be drawn to the electrodes, which changes electrode potential. An integrator coupled to the FET transforms the change of the PIN diode's electrode potential into a corresponding change in a voltage across a feedback capacitor.
As X-ray quanta keep hitting the PIN diode one after another, a graph of the capacitor voltage as a function of time starts resembling a stepwise ramp, which gradually approaches a limit of the detector's dynamic range. Before that happens, the accumulated charge from the electrodes of the PIN diode must be neutralised, after which a new ramping period begins and the same steps are repeated. Known techniques for neutralising the accumulated charge include briefly coupling one electrode of the PIN diode to a current source, using an optically active FET that is briefly triggered into a more conductive state by an optical activation pulse, and causing a momentary swing in the bias voltage of the PIN diode. The correct moment for performing the switched neutralisation can be found by comparing the capacitor voltage to a reference, so that reaching the reference triggers the neutralising action.
A drift detector is a newer detector type, with better noise characteristics and consequently better energy resolution than traditional PIN detectors. Said better noise characteristics also allow combining drift detectors with faster processing electronics, which is an advantage. A good overview about certain important aspects of known drift detector technology is given in Carlo Fiorini, Peter Lechner: “Continuous Charge Restoration in Semiconductor Detectors by Means of the Gate-to-Drain Current of the Integrated Front-End JFET”, IEEE Transactions on Nuclear Science, Vol. 46, No. 3, June 1999.
FIG. 1 is a schematic circuit diagram of a known drift detector. What appears as a diode 101 in the diagram is a specific semiconductor detector component that differs e.g. from the PIN diodes of known PIN detectors in that it comprises a field electrode arrangement adapted to control the movements of charge, as well as an integrated amplifier component, essentially a FET 102. The capacitance of the drift detector diode 101 is much smaller than that of a PIN diode. The basic semiconductor material is typically silicon, although other materials are not excluded from consideration. Received X-ray quanta again cause the accumulation of charge as the result of a photoelectric effect. The accumulating charge draws the gate potential of the integrated FET 102 into the negative direction. The FET is coupled to act as a follower, which means that its source potential mimics the change in the gate potential, so that from said source potential a signal may be taken to amplification and detection.
Lowering gate and source potentials in the FET 102 increase the drain-source voltage, which in turn gives rise to increasing currents through the FET 102. A leakage current 103 from the drain to the gate increases, continuously neutralising the accumulating charge that resulted from the detected quanta, until a dynamic equilibrium state is reached, in which the neutralising effect of the leakage current is equal to the mean rate of charge accumulation.
As an inherent problem of the composition of a drift detector comes the character of the neutralising current as a basically random process of moving charge carriers across interfaces between doped regions in a semiconductor. Additionally it should be noted that since hits occur at random in the detector, the magnitude of neutralisation current varies over time. Mathematically it can be shown that a noise term in the overall noise of a drift detector depends on momentary pulse frequency, and thus on the varying value of neutralisation current. In a somewhat simplified manner we may state that the neutralising current is a source of noise, which limits the resolution that can be obtained with a drift detector.
It is not possible to copy the ramp-and-neutralise cycle described above from PIN detector applications to drift detectors, because although in principle eliminating the continuous noisy neutralisation current, it would just introduce another error source. The stray capacitances inherent to a FET change as a function of voltage, which would cause the indication of the energy of detected quanta depend on whether they were detected at the beginning of or close to the end of the ramp. In other words, applying the ramp-and-neutralise cycle would spread an energy peak obtained as an output of the detector in a relatively hardly predictable manner.
An objective of the present invention is to present a detector appliance based on a drift detector in which the noise-introducing effect of neutralisation current is reduced. An additional objective of the invention is to present a method for operating a drift detector with reduced noise-introducing effect from neutralisation current. A yet another objective of the invention is to provide a detector appliance for detecting quanta of electromagnetic radiation with fast response and good energy resolution.
The objectives of the invention are achieved by rapidly shooting a pulse of neutralising current to the detector element of a drift detector repeatedly after one or only a few quanta have been measured.
According to a first aspect of the invention, a detector applicance comprises:                a semiconductor detector component adapted to be exposed to electromagnetic radiation,        a amplifier component integrated with said semiconductor detector component to form a drift detector,        a neutralising current path for conducting a neutralising current through said amplifier component to said semiconductor detector component and        a switch coupled to said neutralising current path, said switch being adapted to control the flowing of said neutralising current.        
According to a second aspect of the invention, a detector appliance comprises:                a drift detector chip comprising a detector diode and an integrated field-effect transistor adapted to act as an amplifying component,        a preamplifier with an input coupled to a source electrode of said integrated field-effect transistor and an output,        a linear amplifier having an input coupled to the output of said preamplifier and comprising a timing channel adapted to produce a timing pulse,        a latch circuit coupled to receive a timing pulse from said timing channel and adapted to sample a received timing pulse and to temporarily store a sampled received timing pulse,        a neutralisation current switch coupled between said latch circuit and said integrated field-effect transistor,        a latch emptying switch coupled between said latch circuit and a fixed potential, and        a pulse generator having an input coupled to receive a timing pulse from said timing channel, a first output coupled to control a state of conduction of said neutralisation current switch and a second output coupled to control a state of conduction of said latch emptying switch;        
wherein said pulse generator is adapted to respond to receiving a timing pulse by first setting said neutralisation current switch into conductive state and thereafter setting said neutralisation current switch into nonconductive state and said latch emptying switch into conductive state.
According to a third aspect of the invention, a detector appliance comprises:                a drift detector chip comprising a detector diode and an integrated field-effect transistor adapted to act as an amplifying component,        a preamplifier with an input coupled to a source electrode of said integrated field-effect transistor and an output,        a linear amplifier having an input coupled to the output of said preamplifier and being adapted to produce an amplified pulse indicative of a hit of a quantum being detected in the detector diode,        a neutralisation current switch coupled between a neutralisation current source and said integrated field-effect transistor, and        a timer having an output coupled to control a state of conduction of said neutralisation current switch;        wherein said timer is adapted to repeatedly set said neutralisation current switch into conductive state and thereafter into non-conductive state.        
According to a fourth aspect of the invention a method for neutralising accumulated charge in a drift detector comprises:                producing an indication of an occurred hit of a quantum in the drift detector, and        based on said indication, triggering a pulse of deliberately increased neutralisation current into the drift detector for the duration of a limited time interval.        
According to a fifth aspect of the invention a method for neutralising accumulated charge in a drift detector comprises:                regularly triggering a pulse of deliberately increased neutralisation current into the drift detector for the duration of a limited time interval, and        tuning an operational characteristic of such regular triggering in proportion to monitored accumulation of charge in the drift detector, the operational characterstic being at least one of the following: an amount of how much said neutralisation current is increased, a frequency at which triggering occurs, a duty cycle of triggered pulses of deliberately increased neutralisation current.        
A synchronised neutralising strategy according to an embodiment of the invention is such where individual hits of quanta are observed, and a rapid, neutralising current pulse is shot to the detector element after each hit or after a small number of consecutive hits. An alternative, unsynchronised neutralising strategy according to another embodiment of the invention is such where synchronisation is performed regularly in cycles short enough to only allow a small number of hits between consecutive neutralisation rounds.
The structure and operation of typical known drift detectors already involves producing a so-called timing pulse as a response to an individual quantum hitting the detector. The purpose of the timing pulse is to act as an accurate indicator of the moment of time when the hit occurred. The height or amplitude of a timing pulse is not an accurate indicator of quantum energy, but has some approximate correspondence therewith. It is thus possible to use a slightly delayed timing pulse to trigger a neutralising event after each individual hit at the detector, or to collect a few consecutive timing pulses and neutralise their combined charge accumulation effect in a collective shot. Such action constitutes synchronised neutralisation.
If the accuracy at which the amplitude of a timing pulse approximates detected quantum energy is not enough, it is possible to use fine tuning to more accurately determine the amount of neutralisation current to be administrated in each shot. An advantageous fine tuning arrangement utilises a feedback loop, which monitors a voltage level indicative of accumulated charge at the FET and controls accordingly the amplification factor of a controllable amplifier, through which the neutralisation current is conveyed. Basically it would also be possible to obtain an indication of the accurate quantum energy from the actual measurement channel, where a measurement pulse is formed parallelly with but with more accuracy than the timing pulse.
In unsynchronised or asynchronous neutralisation the timing pulses are not necessarily needed to trigger shots of neutralisation current, because the last-mentioned are delivered according to a fixed time schedule. However, in order to ensure that not too many hits will occur between consecutive neutralisation rounds, it may be advantageous to monitor at least the frequency of timing pulses and to adaptively set the neutralisation frequency so that it is not smaller than a predetermined fraction of the frequency at which hits occur in the detector.
The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb “to comprise” is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.